1. Field of the Invention
The present invention relates to a sensor circuit and to a method of producing it and, in particular, to a magnetic sensor circuit having a sensor element the output signal of which depends on an external magnetic field, a current source for providing an operating current for the sensor element and an amplifier circuit for amplifying a sensor voltage produced by the sensor element.
2. Description of the Related Art
Monolithically integrated magnetic sensors, such as, for example Hall sensors, magnetoresistive sensors and so-called giant magnetoresistive sensors, are commercially available and are increasingly utilized, apart from their actual usage as measuring devices for magnetic fields, where a contactless switching is to be effected. Such a field is, for example, revolution counting in the gear or tachometer area of motor vehicles. The Hall elements of such monolithically integrated Hall sensors have sensitivity differences due to technology and temperature.
For compensating sensitivity differences due to technology and temperature, EP 0525235 B1 discloses a Hall sensor which is to enable self-compensation of sensitivity differences without individual equalization measures. For this purpose, the operating current of the Hall element in this Hall sensor is provided by a band gap circuit. The band gap circuit represents a first and a second current source producing a first and a second auxiliary current through a first and a second resistor. The semiconductor zones of the first and the second resistor are produced in essentially the same way as the semiconductor zone of the Hall element. In addition, a first and a second current converter are provided supplying a first and a second partial current having fixed pre-given transforming ratios regarding the first and the second auxiliary current. Finally, adding/subtracting means are provided to produce operating currents for the Hall elements with the desired temperature dependencies by a sum/difference formation of the first and second partial currents.
Further possibilities for compensating sensitivity differences due to technology and temperature of Hall elements are to use several such band gap circuits. In addition, equalizing can for example take place using laser fuses on the wafer on which the Hall sensor circuit is formed or by means of an EEPROM equalization after packaging. Finally, the Hall elements can be measured, wherein such Hall elements not satisfying a predetermined specification are sorted out, which has the consequence of a yield loss.
It is a disadvantage of the bias system of EP0525235B1 described above, which bases on resistors formed like the Hall element that such resistors have a large area and large leakage currents at high temperatures. According to EP 0525235 B1, the currents produced by the band gap circuit are additionally used to determine the switching points of a comparator evaluating the amplified Hall voltage. Thus, there is a high temperature coefficient in both the resistors themselves and the currents determining the switching points.